1. Field of the Invention
The present invention relates to a method for inspecting a tubular, and more particularly relates to a method for inspecting coiled tubing using ultrasound.
2. Related art
Coiled tubing is pipe stored on a reel known to be useful for a variety of applications. In particular, coiled tubing is useful for being run into and pulled out of a bore, tubular string, borehole, pipeline, well or wellbore to accomplish desired operations. In use, the coiled tubing is unwound from the coiled tubing reel and fed or injected into a borehole. After completing a downhole use or operation, the coiled tubing is withdrawn from the borehole and rewound on the reel. The repeated bending caused by unwinding and rewinding the coiled tubing can degrade or damage the coiled tubing. Other factors, such as the pressures to which the coiled tubing is subjected, the number of times the coiled tubing is run into and pulled out of the borehole, and the type and configuration of equipment used to deploy and retrieve the coiled tubing from the borehole, also stress the coiled tubing in a variety of ways that can degrade or damage it. This damage, called fatigue damage, can cause the coiled tubing to weaken and ultimately fail. Fatigue damage limits the useful life of coiled tubing. Coiled tubing parameters that may indicate fatigue damage are monitored to confirm that coiled tubing remains in suitable and safe condition for use in a given job environment. Monitoring of parameters such as number of cycles of use; radii of bending, forces applied to the coiled tubing; length, diameter, and wall thickness of the coiled tubing; material properties; and internal pressures is useful in predicting the useful life of coiled tubing. In particular, monitoring of the wall thickness of the coiled tubing is important as it can be affected by corrosive or erosive fluids or slurries pumped through it as well as fatigue stresses.
One method known for measuring or monitoring parameters of tubulars is ultrasonic testing. For example, U.S. Pat. No. 5,600,069, incorporated herein by reference, presents an ultrasonic assembly and method of testing useful for measuring tubing parameters as a manufacturing quality confirmation test. Methods of ultrasonic testing more particularly adapted for detecting inner surface flaws near the ends of tubulars and across welds are described in U.S. Pat. No. 6,578,422, incorporated herein by reference.
Ultrasonic testing devices for coiled tubing are known. U.S. Pat. No. 5,303,592 incorporated herein in its entirety by reference, describes an ultrasonic apparatus and methods of use for inspecting coiled tubing wherein coiled tubing is passed through a cylindrical test apparatus coupled to the coiled tubing while ultrasonic signals are transmitted into and returned from the coiled tubing. The success of ultrasonic testing of coiled tubing depends on the ability to transmit a signal from the ultrasound transducers to the coiled tubing. Water may be used to provide coupling between the ultrasonic transducers and the coiled tubing as described in U.S. Pat No. 5,303,592. However, other fluids and debris in a borehole environment can contaminate and dirty water, which interferes with its ability to relay the signals transmitted from the ultrasonic transducers and returned from the coiled tubing.
Another approach such as described in WO 2004/025291 involves an apparatus and method of transmitting and receiving ultrasonic signals to and from the coiled tubing through an elastomeric element coupled with the tubing. The elastomeric element material may be cylindrical, surrounding the coiled tubing around its circumference, over a certain axial length. Ultrasonic transducers, capable of transmitting and receiving acoustic signals, are placed around the elastomeric element, generally along its exterior circumference. The elastomeric element, transducers, and other mechanisms may be provided in a housing. Such a housing may also provide a means to mount or locate the apparatus near the injector device during operational use.
The quality of the ultrasonic signal provided to and returned from the coiled tubing greatly depends on the coupling between the elastomeric element and the coiled tubing. While it is desirable that the elastomeric element be firmly pressed against the coiled tubing for optimal signal quality, firm contact of the element with the coiled tubing generates friction as the coiled tubing moves through the apparatus. Such friction can generate heat that can negatively affect the properties of the elastomeric material as well as the coiled tubing. Furthermore, in the scope of the overall operation of deploying and retrieving coiled tubing from a borehole, it is also desirable that minimal constrictions be placed on the coiled tubing to avoid unnecessary friction as it is moved in and out of the borehole to avoid slowing down the job site operations.
It is suggested in WO 2004/025291 to provide a fluid such as oil between the elastomeric material and the coiled tubing to improve sonic coupling. In addition to the disadvantages of using water for coupling, use of other fluids or chemical may simply not be permitted for use in sensitive environments. When a fluid becomes dirty during borehole use, quality of the received signals can be affected. In addition, while use of a lubricant may reduce the friction created by the contact of the elastomeric element with the moving coiled tubing for a certain period of time, as operations proceed, the lubricant will be worn away or degrade under borehole conditions, leading to eventual friction buildup. Finally, it is not clear how the presence and acceptable condition of such a lubricating and coupling fluid would be confirmed during operational use, other than by measuring a frictional induced increase in temperature or drag, that friction itself being indicative of a reduction in the lubricating performance of the fluid.
It is desirable to necessary to accomplish the acoustic coupling between ultrasound transmitters and the coiled tubing that is required for signal quality in such a manner that minimizes friction on the coiled tubing as it is moved in and out of a bore. Further, as it is important from both safety and operational efficiency that the coiled tubing parameters be accurately determined to permit appropriate monitoring of the tubing condition, the method of acquiring such parameter data must be reliable and confirmable. There exists a need for a measurement process that is reliable and confirmable during operational use. Furthermore, there exists a need for a method and apparatus that provides good quality signal transmission for measuring tubing parameters while avoiding or minimizing creating friction during the measurement.